Electrolytic hydrodimerisation

ABSTRACT

ETHYLENICALLY UNSATURATED MONOMERS OF THE FORMULA R1R2C=CR3Z, WHERE R1, R2 AND R3 EACH REPRESENTS A HYDROGEN ATOM OR A HYDROCARBON RADICAL AND Z REPRESENTS A KETONE, ACID, ESTER, NITRILE, AMIDE OR ALDEHYDE GROUP, ARE HYDRODIMERISED BY CATHODIC REDUCTION OF A SOLUTION COMPRISING THE MONOMER, WATER AND A BUFFER OF PH 6 TO 10 CONTAINING AT LEAST ONE QUATERNARY AMMONIUM SALT OF AN OXYGEN-CONTAINING INORGANIC ACID HAVING A PK IN WATER OF FROM 5 TO 11.

United States Patent 3,556,961 ELECTROLYTIC HYDRODIMERISATION Jean Bizot, Thiais, Guy Bourat, Bourg-la-Reine, and

Daniel Michelet, Lyon, France, assignors to Rhone- Poulenc S.A., Paris, France, a French body corporate No Drawing. Filed June 29, 1967, Ser. No. 649,846 Claims priority, application France, June 30, 1966, 6 3

Int. C1. C07 1/00 U.S. Cl. 204--73 12 Claims ABSTRACT OF THE DISCLOSURE This invention relates to the hydrodimerisation, by electrolytic means, of cap-ethylenically unsaturated monomers, more particularly acrylonitrile.

It has been known for a long time to carry out the hydrodimerisation of compounds having a double bond either by chemical means or by an electrolytic process. Thus, for example, sorbic and cirmamic acids, u,,B-unsaturated ketones or compounds such as coumarin, stilbene, and acrolein, have been hydrodimerised by electrolysis of their aqueous solutions, optionally in the presence of cosolvents (see Wilson, Trans. Electrochem. Soc. 80, 139 (1941) and 84, 153 (1943); Pasternak, Helv. Chem. Acta 31, 753 (1953); Knounjants, Usp. Khim, 23, (7) 781-820 (1954); and Tomilov, Russian Chem. Rev. 32, 36-37). It was in 1946 (see U.S. patent specification No. 2,439,308) that the preparation of adiponitrile from acrylonitrile by chemical means was first achieved. Some years later Kern succeeded in polymerising acrylonitrile by electrolysis in an aqueous medium (see Kern, Makromolekulare Chemie, 10, 204 [1953]), and in 1954 Knounjants hydrodimerised acrylonitrile by indirect electrochemical reduction in an acid medium at a mercury cathode [see Chem. Abs. 51, 11133 (1957)].

From a technical point of view it is also known that the electrolytic reduction potential of acrylonitrile is of the order of 2 v. relative to a calomel electrode [see W. L. Bird, Anal. Chem. XXIV 586 (1952)] and that for electrolytic reduction it is essential that the electrolyte should not be reduced before the compound which one wishes to reduce. By using electrolytes based on quaternary ammonium salts it is possible to achieve sufliciently negative potentials [Bird, Anal. Chem. 24, 586 (1952); Strackelberg, Z. fiir elektrochem. 53,118 (1949)]. Thus, for example, the hydrodimerisation of acrylonitrile in the presence of a quaternary ammonium arylsulphonate has been carried out (see French patent specification No. 1,328,327), but the quaternary ammonium salts hitherto used suffer from certain disadvantages. Thus, the quaternary ammonium sulphonates and halides frequently degrade at the anode and can at times yield corrosive compounds. It is therefore often necessary to use a separator (a porous body or ion exchange membrane) between the anode and cathode compartments (see U.S. patent specification No. 3,193,480), but this results in technological difliculties and waste of power in carrying out the electrolysis.

The present invention provides a process for the hydroice dimerisation of an ethylenically unsaturated monomer of the general formula:

wherein R R and R which may be the same or different, each represents a hydrogen atom or a hydrocarbon radical, e.g. of l to 6 carbon atoms, and Z represents a ketone, ester, nitrile, amide or aldehyde group, which comprises electrolysing a mixture of (a) the monomer, (b) water, the ratio of monomer/water being from 15/85 to /25 by weight, and 10 to 30%, preferably from 15 to 25%, based on the total weight of the mixture of (c) a buffer containing at least one quaternary ammonium salt of an oxygen-containing inorganic acid having a pK in water of from 5 to 11, preferably from 7 to 10, the components of the buffer being in such a ratio that the pH of their aqueous solution is from 6 to 10. The preferred ethylenically unsaturated monomer is acrylonitrile. The process is particularly advantageous in that the electrolysis can be carried out without a separating membrane, concentrated aqueous solutions are used, and adiponitrile is obtained in a high degree of purity from acrylonitrile. Preferably the weight ratio of water/monomer in the homogeneous mixture is from 60/40 to 35/65.

The anions of the butters used in the process of this invention are for example anions of weak acids such as borates, carbonates or bicarbonates, and phosphates which are not destroyed at the anode. They have a buffering effect this term being understood as the property of an aqueous solution, which contains an acid-base couple in the Bronsted sense, of varying little in pH whilst considerable quantities of acid or base are added to it. The pH of the aqueous solution is thus stabilised within a range on either side of the value of the pK of the acid-base couple. Acidbase couples suitable for use in the process of the present invention are those whose pK is between 5 and 11 and more particularly between 7 and 10, such as, for example, the monobasic phosphatedibasic phosphate, borate-boric acid or carbonate-bicarbonate couples, in amounts such that the pH of their aqueous solution is between 9 and 10.

The buffering effect is a maximum when the basic form and the acid form are present in equal concentrations. Nevertheless these proportions can be changed if a pH other than the pK of the couple is desired, without however one of the forms being too much in excess relative to the other. The best results are obtained when the acid form represents a concentration which is 25 to 75% of the concentration of the salt. Salts derived from a weak acid having a pK of between 5 and 11 and therefore having a buffering effect relative to the pH in an aqueous solution, but which would be oxidised or otherwise degraded at the anode may be used in the process of the invention though they are of less interest since their use requires separation of the anode and cathode compartments.

It is found in all cases that the presence of the buffer makes it possible to inhibit the formation of the usual byproducts such as, for example, propionitrile, p-hydroxypropionitrile or bis (B-cyanoethyD-ether.

The use of tertiary amine salts which can have a certain buffering effect has been proposed previously, but they are reduced at a potential close to that necessary for the hydrodimerisation and this makes them of little value in most cases.

The cations which are suitable for the process of the present invention must impart a strong hydrotropic effect to the electrolyte. The tetra(n-butyl) ammonium cation is particularly suitable. Solutions of acrylonitrile in water containing 75 of acrylonitrile by weight and less than 30% of salt, relative to the total of the three components,

can be made up, and provided at least by weight of the tetrabutyl ammonium salt (such as the carbonate) is used, it will be miscible in all proportions with water and aerylonitrile.

Salts or mixtures of salts which are particularly suitable for use in the process of the present invention are for example equimolecular mixtures of the tetra-(n-butyl) ammonium monobasic phosphate and the tetra(n-butyl) ammonium dibasic phosphate, Whose pH in aqueous solution is 7.4. By increasing the proportion of the dibasie salt the pH in water can be fixed at a value of between 7.5 and 8.5. Equally, mixtures of boric acid and tetra(nbutyl) ammonium borate (which makes it possible to stabilise the pH in water at about 9) and especially mixtures of tetra(n-butyl) ammonium or tetra(n-propyl) ammonium carbonates and bicarbonates can be used. In the latter case the proportions used are such that the resulting pH in water is preferably between 8.5 and 10.

For the hydrodimerisation of acrylonitrile, the acrylonitrile content is normally from to 75% by weight based on the total weight of the solution and preferably from to 50% by weight. A mixture which is particularly suitable for the hydrodimerisation of acrylonitrile contains 50% by weight of acrylonitrile, 25% by weight of water and 25 by weight of one of the above-mentioned mixtures of tetra-(n-butyl) ammonium salts.

The electrolysis is generally carried out in a cell with a single compartment, although cells having separate compartments may also be used. However the low electrolysis potential required in the process without separation makes it possible to achieve electrical energy yields (kWh. per kg. of adiponitrile produced) which are very much greater than those achieved using the process with separation.

The cathodes which can be used are those which make it possible electrochemically to reduce compounds which are difi'icult to reduce without interference by the reduction of the water. Mercury, graphite and lead, which have previously been used, provide suitable cathodes, but it has been found that alloys of lead with mercury, and Darcet alloy, also give good results.

Suitable anodes are those which have a low oxygen overpotential in the electrolysis of water, such as, for example, lead anodes which may or may not be coated with oxide, nickel which may or may not be surfaceoxidised, platinum deposited onto graphite, gold, or stainless steel which has preferably been made passive. Insoluble anodes which have an oxygen overpotential less than that of gold are preferably used.

The potential which has to be applied between the anode and the cathode in order to carry out the process of the present invention is generally low. The ohmic potential drop, and correspondingly the energy loss by the Joule effect, in the electrolyte can be reduced by bringing the electrodes closer together. This distance, however, is not limited but in order to have good circulation of liquid between the electrodes their distance apart may advantageously be from 1 to 15 millimetres and preferably from 1 to 3 millimetres. The potentials to be applied are chosen so as to have a suitable cathode potential, and potentials of 3 to 8 volts are generally very suitable for a single-compartment cell.

The electrolytic bath is moderately stirred and the electrolysis is preferably stopped when about half the product has been converted for a discontinuous process.

When aerylonitrile is hydrodimerised according to the process of the present invention, practically no by-products, such as propionitrile, h-hydroxypropionitrile or bis- (B-cyanoethyhether, form. The electrical yields are generally above 90%.

The following examples illustrate the invention. While they relate to acrylonitrile, they apply also, mutatis mutandis, to compounds such as a,fi-ethylenically unsaturated aldenhydes, ketones, and derivatives of 0;,[3- ethylenically unsaturated acids such as amides.

4 EXAMPLE 1 The apparatus comprises a single-compartment cell equipped with a reflux condenser, a mercury cathode of cm. surface area and a lead anode in the form of a perforated plate of area 60 cm. arranged parallel to the surface of the mercury and 10 mm. therefrom. The mercury and the electrolytic solution are stirred.

A butter solution is prepared by bubbling carbon dioxide into an aqueous solution containing 40% by weight of tetra (n-butyl) ammonium hydroxide until the pH of the solution is 9. The solution then contains about 500 g./l. of a mixture of tetra-(n-butyD ammonium carbonate and bicarbonate. 5 3 g. of this solution are mixed with 53 g. of acrylonitrile and 27 g. of water and the combined solution is introduced into the cell. The potential at the terminals is adjusted to 6 v. and the temperature to 40 C. 42,500 coulombs are passed across the cell with moderate stirring of the mercury and the electrolytic solution. The cell is then emptied, rinsed with methanol, and 149 g. of a practically colourless solution are thus obtained.

Analysis shows that the solution contains 15.2% by weight of adiponitrile, which has thus been obtained in an electrical yield of 95.3%. Neither propionitrile, fi-hydroxypropionitrile nor bis-(fi-cyanoethyDether are de tected in the product by gas-liquid chromatography (sensitivity threshold of the method below 0.2%).

EXAMPLE 2 An electroyte obtained by mixing equal amounts of acrylonitrile and an aqueous solution containing 500 g./l. of a mixture of tetra(n-butyl) ammonium carbonate and bicarbonate and having a pH of 9 (prepared as described in Example 1) is introduced into a closed cell under reflux at ambient temperature, the cell having a single compartment provided with a mercury cathode and an anode consisting of a spiral of pure nickel wire previously oxidised with air in a flame.

The process is carried out at ambient temperature. 2,900 coulombs are passed whilst keeping the potential of the mercury at about 2 v. relative to the calomel electrode. 9.5 g. of a very pale yellow solution containing 15.8% of adiponitrile and 0.1% of propionitrile are collected. Neither bis-(,S-cyanoethyl) ether nor fl-hydroxypropionitrile are detected by gas-liquid chromatography. The electrical yield is thus 82%.

EXAMPLE 3 A closed electrolysis cell under reflux is provided with a mercury cathode, a platinum anode and a commercial cation exchange membrane based on sulphonated polyethylene for separating the compartments. The process is carried out at ambient temperature.

10 g. of acrylonitrile and 10 g. of an aqueous solution of a tetra(n-butyl) ammonium carbonate-bicarbonate butter containing 500 g. of salt/l. and of pH9 (prepared as described in Example 1) are introduced into the cathode compartment. 7 N sulphuric acid is introduced into the anode compartment. The potential is adjusted to 30 v. Whilst maintaining the potential of the mercury cathode at l.9 v. relative to the calomel electrode, 5323 coulombs are passed through the cell. The cathode compartment is then emptied, rinsed with a little methanol and the two solutions are mixed to give a single solution weighing 23.1 g.

Analysis shows that this solution contains 12.6% by weight of adiponitrile. Neither propionitrile, fi-hydroxypropionitrile nor bis-(fi-cyanoethyl) etherare detected in the product by gas-liquid chromatography. The adiponitrile was thus produced in an electrical yield of 98%.

EXAMPLE 4 A closed electrolysis cell under reflux having a single compartment is provided with an anode consisting of a lead wire of diameter 5 mm. and length 3 cm. The

cathode is a similar wire of length 5 cm., which has been surface-amalgamated by immersing it in mercury for 1 minute, followed by careful wiping. The process is carried out at ambient temperature.

The electrolytic bath consists of 6 g. of an aqueous solution of a tetra-(n-butyl)ammonium carbonate-bicarbonate buffer (prepared as described in Example 1) to which 6 g. of acrlyonitrile and 5 g. of Water were added. The electrolysis is stopped after 2,147 coulombs have been passed through the cell at 5 volts.

Analysis shows that the solution then contains 1.2 g. of adiponitrile which has thus been synthesised in an electrical yield of 99.8%. Neither propionitrile, bis(;3-cyano ethyl) ether nor fl-hydroxypropionitrile are detected in the product by gas-liquid chromatography,

EXAMPLE 5 A closed cell with a condenser is made up from a battery of 12 identical lead sheets each 13.25 cm. in area, arranged vertically with a distance between the plates of 2 mm. They are connected to two conductors so that each anode is interposed between two cathodes and each cathode between two anodes. The electrolyte is circulated by means of a pump and passes between the plates, and then into a cooling and degassing chamber kept at a temperature of 20 C.

The electrolytic solution introduced into the cell has the following composition:

'- G. Tetra-(n-butyl) ammonium carbonate-bicarbonate butfer solution (of identical composition to that described in Example 1) 120 Acrylonitrile 120 Water 60 100,000 coulombs are then passed through the cell. The potential applied between the two electrodes is between 4 and 6 v. during the electrolysis.

Analysis of the resulting solution shows that it contains 52 g. of adiponitrile which has thus been synthesised in an electrical yield of 93%.

'EXAMPLE 6 10.25 g. of acrylonitrile and g. of an aqeuous solution of a tetra(n-butyl) ammonium monobasic phosphatedibasic phosphate buffer obtained by neutralising a 40% by weight aqueous solution of tetra-(n-butyl) ammonium hydroxide to pH 8.3 with concentrated phosphoric acid are introduced into the cathode campartment of the separate compartment cell described in Example 3.

4,440 coulombs are passed through the cell whilst maintaining the potential of the mercury cathode at -1.8 v. relative to the calomel electrode. The cathode compartment is emptied and then rinsed with methanol. The combined solutions weigh 22.9 g. and analysis shows that they contain 9% by weight of adiponitrile. The electrical yield is thus 83 Neither propionitrile, bis (fl-cyanoethyl) ether nor B-hydroxypropionitn'le is detected in the product by gas-liquid chromatography.

EXAMPLE 7 The same arrangement as in Example 2 is used but with a platinum rather than an oxidised nickel anode; the electroytic bath is produced by mixing 10 g. of acrylonitrile and 10 g. of a buffer solution of tetra-(n-butyl) ammonium borate and boric acid containing 50% by weight of buffer (pH=9). l

After 2,880 coulombs have been passed, the solution contains 1.57 g. of adiponitrile representing an electrical yield of 97.5%.

EXAMPLE '8 The same arrangement as in Example 2 is used but the nickel anode is replaced by a lead sheet. The electrolytic bath consists of a mixture of 21.8 g". of acrylonitrile and 10 g. of a solution containing 450 g./l. of a mixture of tetra-(n-propyl) ammonium carbonate and bicarbonate. The mixture of these salts is in such a ratio that the pH of their aqueous solution is 9. The potential at the terminals of the cell is 5 v.; after 12,200 coulombs have been passed, the cell is emptied and rinsed with methanol. The combined solution so obtained weighs 40 g. and contains 14.1% by weight of adiponitrile, which has thus been obtained in an electrical yield of 82.5%. Neither propionitrile, fl-hydroxypropionitrile nor bis-( 8- cyanoethyl) ether is detected in the product by gas-liquid chromatography.

.2' EXAMPLE 9 The procedure of Example 8 is followed, the electrolytic bath consisting of a mixture of 17.3 g. of acrylonitrile, 10.5 :g. of water and 11.4 g. of an aqueous solution containing 589 g./litre of a mixture of tetra-(n-hexyl) ammonium carbonate and bicarbonate. The mixture of these salts is in a ratio such that the pH of their aqueous solution is 9.4. The potential at the terminals of the cell is 8 v.; after 14,500 coulombs have been passed, the cell is emptied and rinsed with methanol. The combined solution so obtained weighs 44 g. and contains 14.5% by weight ofgadiponitrile which has thus been obtained in an electrical yield of 78.7%. Neither propiontrile, fi-hydroxypropionitrile nor bis-(fi-cyanoethyl) ether is detected in the product by gas-liquid chromatography.

EXAMPLE 10 The procedure of Example '8 is followed. The electrolytic bath? consists of a mixture of 7.7 g. of acrylonitrile, 2.25 g. of water and 5 g. of a solution containing 680 g./litre of a mixture of tri(n-butyl)n-propyl ammonium carbonate and bicarbonate, the mixture of these salts being in a ratio such that the pH of their aqueous solution is 9.The potential at the terminals of the cell is 7 v.; after 6,900 coulombs have been passed, the cell is emptied and rinsed with methanol. Analysis shows that the combined solution contains 3.04 g. of adiponitrile which has thus been obtained in an electrical yield of 78.7%. Neither propionitrile, fi-hydroxypropionitrile nor bis-(B-cyanoethyl) ether is detected by gas-liquid chromatography.

We claim:

1. In a process for the hydrodimerisation of an ethylenically unsaturated monomer of the formula:

wherein R R and IR which may be the same or difi'erent, each represents a hydrogen atom or a hydrocarbon radical and Z represents a ketone, ester, nitrile, amide or aldehyde group, by the electrolysis, between a cathode at which the said monomer is reduced at a potential lower than that required to reduce water and an anode having an oxygen over-potential less than that of gold, of a mixtureof (a) the said monomer, and (b) water, the ratio of monomer/Water being from 15/85 to 75/25 by weight, the improvement which comprises including in the said mixture, as buffer, 10 to 30%, based on the total weight of the mixture, of (c) a quaternary ammonium salt of an oxygen-containing inorganic acid having a pK in water of from 5 to 11, such as to provide such a pH of the said mixture of from 6 to 10, and effecting the electrolysis in a single compartment cell.

2. The improvement according to claim 1 in which the monomer is acrylonitrile.

3. The improvement according to claim 1 in which the ratio of water/monomer is between 60/40 and 35/65.

4. The improvement according to claim 1 in which the electrolyte content is from 15 to 25%.

5. The improvement according to claim 1 in which the pK value of the oxygen-containing inorganic acid is from 7 to 10.

6. The improvement according to claim 1 in which the pH of the aqueous buffer solution is from 8.5 to 10.

7. The improvement according to claim 1 in which the quaternary ammonium salt is tetra(n-butyl) ammonium salt.

8. The improvement according to claim 1 in which the oxygen-containing inorganic acid is carbonic, boric or orthophosphoric acid.

9. The improvement according to claim 1 in which the monomer content, based on the total weight of the solution, is from 25 to 50% by Weight.

10. The improvement according to claim 1 in which the cathode is of a lead/mercury alloy, Daroet alloy, graphite, or mercury.

11. The improvement according to claim 1 in which the anode is of lead, nickel, platinum deposited on graphite gold or stainless steel which has been made passive.

12. The improvement according to claim 1 in which the separation between the electrodes is from 1 to 3 millimetres.

References Cited UNITED STATES PATENTS 3,477,923 7/19 66 Beck et a1. 204--73 3,193,480 7/1965 Baizer et al 20473 3,193,481 7/1965 Baizer 20473 JOHN H. MACK, Primary Examiner R. L. ANDREWS, Assistant Examiner 

